GCN Circular 36355

A.J. Levan (Radboud), J. van Dalen (Radboud), P. Jonker (Radboud), D.B. Malesani (DAWN/NBI & Radboud), J. Quirola-Vasquez (Radboud), F.E. Bauer (PUC) report for a larger collaboration: 

We have obtained further monitoring of the counterpart of EP240414a (Lian et al. GCN 36091
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, Aryan et al. GCN 36094
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) with the VLT, GTC, SOAR, and NOT. The optical source has decayed to an approximately constant i-band magnitude of i~22 but has declined more rapidly in the g-band, with the g-i colour changing from ~0.4 at four days to g-i~2 at 15 days post-burst. The current magnitudes and colours are consistent with a supernova component now visible in the light curve. 

Furthermore, an X-shooter spectrum obtained on 25 April 2024 is not the power-law spectrum expected of a GRB afterglow and shows, at low signal-to-noise, features consistent with a broad-lined type Ic supernovae at z~0.4, the redshift of the putative host galaxy (Jonker et al. GCN 36110
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). Based on the combined light curve and spectral evidence, we suggest that EP240414a is associated with a massive star collapse at a relatively unusual location within its host galaxy. 

We also note that the peak luminosity and lightcurve shape over the time frame 2-8 days after the transient onset consists of a rise and fall whose shape and luminosity are reminiscent of the fast blue optical transient AT2018cow
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. However, limited X-ray coverage makes tracking the afterglow behaviour challenging. 

We thank the staff of the NOT, GTC, SOAR and the VLT for their excellent support.